The combined-income formula
The IRS does not tax Social Security by looking at the benefit alone. Publication 915 has you add half of your benefits to all your other income, including tax-exempt interest, to get what it calls combined income. That figure is compared with a base amount: $25,000 for a single person, head of household or qualifying surviving spouse, $32,000 on a joint return, and $0 for a married person filing separately who lived with their spouse at any point in the year. Below the base amount none of the benefit is taxable.
Between the base amount and an adjusted base amount ($34,000 single, $44,000 joint), up to 50% of the excess becomes taxable, limited to half the benefits. Above the adjusted base amount, 85% of the further excess is added, and the total can never exceed 85% of the benefits. These thresholds were set in 1983 and 1993 and are not indexed for inflation, which is why a larger share of retirees pays tax on benefits every year.
The IRS example
Publication 915 works through a single retiree with $5,980 of net benefits and $28,990 of other income. Half the benefits is $2,990; combined income is $31,980, which is $6,980 over the $25,000 base. Half of that excess is $3,490, but the taxable amount cannot exceed half the benefits, so $2,990 goes on Form 1040 line 6b. The calculator reproduces that example exactly and prints each worksheet line so you can follow the same path with your own numbers.
Then the actual tax
Taxable benefits are only part of the answer; what matters is the tax on your whole return. The calculator adds the taxable benefits to your other income, subtracts the standard deduction, the additional amount for filers 65 or older, and, for 2025 through 2028, the $6,000 senior deduction, which shrinks by 6 cents per dollar of income over $75,000 ($150,000 joint). Many retirees with modest pensions find the deductions cover everything and the tax is zero even though some benefits were "taxable" on paper.
Why a small withdrawal can cost more than its bracket
Inside the phase-in range, each extra dollar of IRA withdrawal adds a dollar of income and makes up to 85 cents more of your benefits taxable. A retiree in the 12% bracket can face an effective rate of 22% on that dollar (12% × 1.85). Running the calculator with and without a planned withdrawal shows the real cost, which is the number that matters when choosing between a traditional IRA withdrawal and a Roth one, or when timing a capital gain.
What is not modeled
Adjustments to income (an IRA deduction, student loan interest, the self-employment tax deduction) reduce combined income slightly and are not entered here. Lump-sum benefits for earlier years have their own election. Withholding on benefits (Form W-4V) is a payment choice, not a tax rule. And the states that tax benefits use their own thresholds. Most do not tax them at all.
Official sources used on this page
- Publication 915 (2025), Social Security and Equivalent Railroad Retirement Benefits: base amounts, Worksheet 1 and its example: 2025
- IRS: Working Families Tax Cuts: the $6,000 deduction for seniors: updated July 25, 2025
- Rev. Proc. 2025-32 (2026 standard deduction, age-65 addition and brackets): 2025-10-09
Every rule record names its source, revision and the date it was last verified; the results panel prints the ones your calculation used. Rules last verified 2 September 2026.